1. Field of the Invention
The present invention relates to the field of apparatus and method for detecting anomalies in ferrous pipe structures, such as natural gas or oil pipelines, through the use of magnetic sensors which are passed along the interior of the pipe structures.
2. The Prior Art
Piping systems, such as natural gas or oil piping systems, need to be inspected, during and after construction, and periodically after use has begun, for the purposes of detecting defects or points of failure or leakage, or in some cases, predicting such points of failure or leakage. Since such pipelines typically are buried or submerged, it has become necessary to develop a means for inspecting such pipe structures using preprogrammed robotic or remotely operated or teleoperated devices.
It is known, for example, that in ferrous pipe structures, such as are used for natural gas or petroleum, the pipe will have a residual or remnant magnetic field associated with it, which can be detected and measured by appropriate electromagnetic sensors placed next to or against the pipe structure. It is also known that by placing magnetic field sources next to such pipe structures, a portion of the magnetic flux from the sources can be forced to travel through the pipe structure. When sensors are activated to seek the imposed field(s) passing through a pipe structure, if the particular local section of pipe is without welds, flaws or other anomalies, then the imposed fields will not be detected. If, however, the local section of pipe has welds, cracks, or other flaws or anomalies, then the imposed field will "leak" from the anomaly and be detected by the sensors, when the leakage field is compared to the profile of the residual or remnant field detected and measured for the same local section of pipe.
Numerous examples of anomaly detection devices, using magnetic field generators and magnetic field sensors, exist in the prior art, including such devices as are disclosed in Beaver et al., U.S. Pat. Nos. 3,460,028; Barton, 3,593,122; Smith, 4,105,972; Birchak et al., 4,649,343; Ando et al., 4,742,298; and Cecco et al., 4,855,676.
The Beaver et al. '028 reference shows an anomaly detector having a single axially oriented magnetic field generating apparatus centrally arranged within the sensor rig. Annular brushes positioned fore and aft of a plurality of magnetic field sensor "sledges" on the sensor rig convey the magnetic field into and out of the pipe structure, such that the magnetic flux lines are parallel to the longitudinal axis of the pipe structure. The "sledges" are held in spring-biased relation against the interior surface of the pipe structure, as the sensor rig moves along the interior of the pipe structure.
The apparatus of the Barton '122 patent likewise employs fore and aft magnetic pole pieces to establish an axially extending magnetic flux path, with sensor "shoes" positioned axially between the pole pieces. An additional pole piece, positioned aft of the paired pole pieces, and having brush parts inclined relative to the interior surface of the pipe structure acts to strengthen the residual magnetic field in the pipe structure. The residual fields detected are compared to the readings taken when the pipe structure has the active magnetic field imposed upon it. The discrepancies in the two sets of readings indicates the presence of anomalies, which may be welds, or actual flaws in the pipe structure.
The Smith '972 patent shows a pipeline inspection vehicle having a plurality of sensor heads arranged in a circle about the vehicle, and held in place against the interior surface of the pipe structure by an annular, spring-biased structure. Each individual sensor head may have magnetic field generating apparatus therein, as well as magnetic field sensing apparatus.
The Birchak et al. '343 patent describes an inspection system for use in small bore tubes, in which a scanner body has two annular magnetic cores, arranged perpendicular to one another, inside a hollow core of the scanner body. An array of magnetic field sensors are arranged circumferentially around the scanner body. The field generated by the two magnetic cores, simultaneously, may be manipulated by phase shifting and amplitude variations, so as to shift the direction of the field, even to producing a helical magnetic flux, or to swing the flux through nearly 180.degree. to expose an anomaly to magnetic flux directed normal to it.
The apparatus of the Ando et al. '298 patent employs an axially extending primary magnetic coil, and a plurality of circumferentially extending secondary coils positioned radially outwardly of the primary coil. The secondary coils do not impose a magnetic field, but rather sense the axial component of the magnetic flux generated by the primary coil, in the form of a voltage imposed on the secondary coil. The sensed component changes in the presence of a flaw positioned between the poles of the primary coil.
The Cecco et al. '676 reference shows an apparatus for inspection of a pipe, having a sensor member configured to produce both axially extending and radially extending magnetic fields, positioned along the length of the sensor member. The Cecco et al. reference describes that both fields are of such strength as to obtain very high levels of saturation in the pipe structure.
The present invention is particularly directed to inspection systems for ferrous pipe structures, which are particularly suited to small bore pipe structures, such as the piping systems for natural gas distribution, the pipes of which typically have a nominal four-inch interior diameter. Commonly, prior art defect detection systems, which have typically been configured for much larger diameter pipes, have relied upon detection techniques involving the complete saturation of the pipe structure by the generated magnetic fields. Such saturation involves substantial power consumption by electromagnets, or the use of large, typically heavy, permanent magnets.
To provide logistical support, such as power cables and transformers or sufficiently powerful and durable portable power supplies for electromagnets, and/or conveyance mechanisms for the sensors for saturation-type detector devices, in the confined environment of a small-bore detector device, is a difficult task. It is desirable, therefore, to provide a way of avoiding the logistical difficulties presented by saturation-type detector systems.
Typically, prior art systems simply measured the magnitudes of the residual/remnant fields and the leakage fields. However, such magnetic fields have other quantifiable and measurable characteristics, such as the phase of the field. It would be desirable to provide an apparatus which can take a wider, more detailed variety of field measurements, rather than taking simple magnitude measurements, as such would enable the use of more compact, smaller powered and lighter sensor apparatus, as would be needed and appropriate for smaller diameter pipe structures.
In addition, prior art devices, such as those mentioned, use as the magnetometers, devices such as Hall Effect sensors, which may lack sufficient sensitivity to detect flaws, in low-power applications, and devices such as coil magnetometers, which require the sensor unit to be in fairly rapid motion in order to obtain readings at all. It would be desirable therefore, to provide a pipeline sensor apparatus capable of high sensitivity even in low power applications, and which can take readings at rest.